Add topic to the router port
Add topic to the router port

get_topic method was added to the LogicalRouterPort
Object but topic was not added to the db obj.
Considering that we are planning to move the
router port to there own table the topic was added
to the router port and was not taken from the
router object

Change-Id: I690cfafc75f312c62a9af131f39c7bf75869213a
Closes-Bug: #1560115

FrankDuan/df_code,openstack/dragonflow,FrankDuan/df_code,openstack/dragonflow,FrankDuan/df_code,openstack/dragonflow